The present invention relates to a ceramic part having at least one small hole as well as a method of manufacturing the same. More particularly, the present invention pertains to a ceramic part having small hole(s) which has neither chipping nor an edge portion and which exhibits excellent strength at high temperatures and under high stresses, as well as a method of manufacturing such a ceramic part.
The ceramic parts according to the present invention is suitable for use as blades (turbine blades and turbine nozzle) of a gas turbine.
Ceramic materials, such a silicon nitride, silicon carbide and partially-stabilized zirconia, are highly heat-resistant, highly wear-resistant, very hard and highly corrosion-resistant, and are thus used as parts of a mechanical components. Due to improvements and adaptation of design, the use of ceramics has been expanding.
In recent years, application of such ceramics to a gas turbine engine, which is a next generation engine, has been drawing attention. Gas turbine engines are rotary engines in which a high-temperature combustion gas is linked directly to a turbine rotor to obtain power. The individual components of the engine other than the combustor, such as a compressor, a turbine rotor and a rotary heat exchanger, are rotary machines. Therefore, gas turbines have advantages in that the exhaust gas thereof is less pollutant, a variety of fuels can be used, and vibrations, noise level and weight of the engine can be reduced.
Although the gas turbine engine has the aforementioned advantages, it has not yet been put into practical use yet because it consumes more fuel than conventional engines. Thus, an improvement in the engine heat efficiency has been the essential issue for the practical application of the gas turbine engine. To achieve an improvement in the engine heat efficiency, an increase in the gas temperature (hereinafter, referred to as a TIT) at the inlet of a turbine is the requirement.
This is the reason why a ceramic gas turbine is the synonym of a gas turbine. Practical application and development of ceramics which are more heat-resistant than heat-resistant alloys have therefore been desired.
However, the use of a ceramic material as a high-temperature gas turbine member under the conditions that TIT exceeds, for example, 1500.degree. C. means that the temperature of the ceramic material partially exceeds 1600.degree. C. The use of a ceramic material under such conditions reduces the strength thereof. Furthermore, due to erosion or corrosion, the reliability and life of the ceramic material as a gas turbine member are reduced.
Under such circumstances, there has been an increasing demand for providing small holes in ceramic parts in order to cool the ceramic part, measure desired data, and so on.
Conventional ceramic parts having small holes are manufactured in the manner described below. After ceramic powder is pressed, the pressed ceramic powder is cold isostatic pressure molded (CIP) and then calcined to remove binder. Thereafter, small holes are formed by dry machining and then the ceramic compact is fired. Alternatively, after firing, the small holes are formed. Normally, small holes are formed by using a drill, ultrasonic waves or a laser.
However, when the small holes are formed by any of the aforementioned methods, a chipped area may be generated around the small hole when the small hole has penetrated the ceramic material. Since a ceramic is a brittle material, generation of chipping greatly reduces the strength thereof, causing breakage at a high temperature and under a high pressure. Particularly, in a ceramic part, such as a combustor nozzle 1 for a ceramic gas turbine, which has a hollow portion 2 and small holes 3 which are opened into the hollow portion 2, as shown in FIG. 5, chipping 4 occurs inside the parts (on the side of the hollow portion), as shown in FIG. 6. Such a chipped area 4 cannot be practically treated, reducing the strength of the ceramic.
Even when no chipping is generated during the formation of the small holes, an edge 5 may be formed around the small hole 3, as shown, in FIG. 7. This leads to generation of chipping and hence breakage of the ceramic due to the stress applied during use.
Furthermore, in a ceramic part having a cooling mechanism which circulates a cooling medium, as the surface roughness of the inner surface of the small hole through which the cooling medium flows increases, the strength of the ceramic greatly reduces because of the brittleness of the ceramic material. When such a ceramic material is used as a component, it may break because of the small hole.
Accordingly, an object of the present invention is to provide a ceramic part having small hole(s) which has neither chipping nor an edge which can cause breakage, and which can thus be strong at high temperatures and under high pressures.